A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of the power switches of the power converter. The duty cycle is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50%). Additionally, as the voltage or the current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value.
To produce a dc output voltage, the power converters often employ diodes to rectify an alternating current (“ac”) voltage produced across a secondary winding of a transformer. The power converter may also employ a diode to provide a current path to provide continuity for a current in an inductor such as an output filter inductor. The aforementioned diode that provides the current path for inductor current continuity is frequently referred to as a “freewheeling diode.” The rectifying and freewheeling devices can introduce a power loss component in a power converter due to the forward voltage drop across the diode, particularly in a power converter that produces an output voltage of five volts or less. Schottky diodes, which have a relatively low forward voltage drop, are often employed in low-voltage power converter applications to reduce a diode forward voltage drop. However, the passive rectifying devices such as Schottky diodes, typically, cannot achieve forward voltage drops of less than about 0.35 volts, thereby limiting a conversion efficiency of the dc-dc power converter.
To achieve an acceptable level of efficiency, dc-dc power converters that provide low output voltages (e.g., 1 volt) often employ rectifying devices that have forward voltage drops of less than about 0.1 volts. To provide such further reduction of power loss, an active semiconductor switch such as a metal-oxide semiconductor field-effect transistor (“MOSFET”), which provides a resistive voltage drop, is often employed to replace the diode. An active semiconductor switch, however, must be periodically driven into conduction and non-conduction modes or states in synchronism with a periodic waveform of an ac voltage (e.g., an ac voltage produced across a secondary winding of a transformer). The active semiconductor switches may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices. A design issue introduced by substituting an active semiconductor switch for a diode is the need to provide a drive signal therefor that is accurately synchronized with the operation of the power converter to control the conduction and non-conduction modes or states of the active semiconductor switches and that avoids conduction overlap with other active semiconductor switches. An active semiconductor switch substituted for a diode in a power converter is generally referred to as a “synchronous rectifier” or “synchronous rectifier switch.”
U.S. Pat. No. 6,060,943, entitled “Circuit Simulating a Diode” to Jansen, issued May 9, 2000 and U.S. Pat. No. 6,469,564, entitled “Circuit Simulating a Diode” to Jansen, issued Oct. 22, 2002, both of which are incorporated herein by reference, are directed to circuits that perform the function of a diode to conduct current in one direction with a low forward voltage drop, but block current in the other direction to produce an improved rectification function. When the voltage at a designated anode terminal of the circuit is higher than the voltage at a designated cathode terminal, a forward current flows. When the polarity of the voltage at these designated terminals is reversed, the current is interrupted.
A number of further circuit design techniques are known in the art to provide a drive signal for a synchronous rectifier. For example, U.S. Pat. No. 5,303,138, entitled “Low Loss Synchronous Rectifier for Application to Clamped-Mode Power Converters,” to Rozman, issued Apr. 12, 1994, which is incorporated herein by reference, discloses that a gate of a synchronous rectifier applied to an active-clamp of a power converter may be driven by a terminal of a secondary winding of a transformer. As described in U.S. Pat. No. 6,288,920, entitled “Drive Compensation Circuit for Synchronous Rectifier and Method of Operating the Same,” to Jacobs, et al., issued Sep. 11, 2001, which is incorporated herein by reference, a drive circuit employing a diode and a capacitor coupled in series with a secondary winding of a transformer is constructed to drive the gate of a synchronous rectifier. As described in U.S. Pat. No. 6,831,847, entitled “Synchronous Rectifier Drive Circuit and Power Supply Including Same,” to Perry, issued Dec. 14, 2004, which is incorporated herein by reference, a drive circuit for a synchronous rectifier is formed with a turn-on switch, a turn-off switch, a charge pump, and a pulse transformer.
Additionally, synchronous rectifier design techniques are described in “Power Supply Cookbook,” second edition, by Marty Brown, which is incorporated herein by reference. As described by Brown in section 3.6.2 therein, FIGUREs (a) and (c) show synchronous rectifiers driven by a primary side switching circuit with a direct connection as in FIGURE (a), and by means of a transformer in FIGURE (c). FIGURE (b) of Brown shows synchronous rectifiers driven directly by the output voltage of a transformer. Thus, as described in the references, a particular power conversion topology including an active clamp may be employed to drive a control terminal of an active semiconductor switch employed as a synchronous rectifier, or an additional transformer winding may be employed for the same.
Each of these approaches, however, provides an efficiency and/or a cost constraint that limits or otherwise penalizes the use of a synchronous rectifier in a many applications. Accordingly, what is needed in the art is a controller for a synchronous rectifier in a power converter and related method that avoids the deficiencies in the prior art.